This invention relates to methods for forming narrow lines of low-resistivity, C54-phase TiSi2 on heavily-doped semiconductors.
Titanium silicide (TiSi2) has been a commonly used metallization in silicon integrated circuits (ICs) due its low resistivity, compatibility with common silicon process steps such as etch, minimal contamination, and ability to reduce the native oxide on silicon.1 However, as feature size shrinks below 0.3 μm, the use of TiSi2 has been decreasing because of an inability to obtain low resistivity TiSi2 on small features. This has been called the “fine line effect.”2 The desire for the continued use of TiSi2 in advanced chip generations has inspired substantial effort to understand and control the fine line effect.
Titanium silicide is typically formed by first sputtering or chemical depositing thin films of Ti on Si. Upon heating, the Ti reacts with the Si to form TiSix phases. The high resistivity C49 TiSi2 phase forms when the film is heated to a temperature between 550 and 700° C. The C49 TiSi2 phase is a base-centered, orthorhombic crystal structure with a resistivity of ˜40–60 μΩcm. This film then transforms to the low resistivity C54 TiSi2 phase upon heating to a temperature in excess of 750° C. The C54 phase is a face-centered, orthorhombic crystal with a resistivity of ˜14–16 μΩcm. Since high-resistivity C49 phase is often unsuitable for Si device performance, conversion of C49 to C54 phase is often important.
Experiments have shown that there is a relatively small density of nucleation sites in the C49 phase film for the C49-to-C54 transformation. As the line width of a structure is decreased, it becomes more difficult to transform the C49 TiSi2 films on top of the narrow lines into C54 phase by thermal annealing due a lack of nuclei. This can result in TiSi2 films in narrow lines that are still in the high-resistivity C49 phase, or a combination of C49 and C54 phases, even after high temperature annealing, with attendant higher resistivity than if the film were completely in the C54 phase. Annealing to temperatures in excess of 850° C. may not induce a complete transformation to C54 phase in narrow lines. Titanium disilicide films annealed at temperatures in excess of 800° C. begin to “thermally groove,” a process by which the individual TiSi2 grains in the film start to become spherical in shape. When thermal grooving is severe, the individual grains separate from one another, making the film discontinuous. The resulting film is no longer conductive.3 
Several factors in addition to narrow line widths influence the C49-to-C54 transformation. Increased annealing temperature is required to induce the C49-to-C54 transformation when: 1) the thickness of the deposited Ti film is decreased; 2) the Si substrate is heavily doped; and 3) the Si substrate is single or polycrystalline, as opposed to amorphous, silicon.2,4 